There are ways round this for the more technically advanced child (mac spoofing), but it's relatively simple to log into most home WiFi routers and block the mac address of the device(s) you want to limit. Most of them also have schedulers that you can put on for this. We do it with the boy's phone, ipad and DS - no WiFi after 9. Of course, his fucking arsehole of a father pays for his data on the phone and doesn't guve a shit that his kid is up until 1 in the bastard morning, so only works when he runs out of 4g towards the end of the month - so we have to physically remove the devices at bedtime.